The present invention relates generally to time-division multiple access satellite communications systems, and more particularly to a technique to be employed by a central station of the TDMA system to receive a burst from terminal stations with a large timing deviation resulting from a satellite drift in orbital position and subsequent bursts arriving at times deviating by small amounts due to variabilities of the operating parameters of the terminal stations.
In the central earth station of a TDMA satellite communications system, the demodulator includes a carrier and clock recovery circuit, which is required to accommodate variabilities between successive bursts received from different terminal stations in a short period of time. These variabilities include carrier frequency deviations, level deviations, clock deviations and burst timing deviations. Control signals are generated at time slot intervals for the carrier and clock recovery circuit, including a resetting signal for initializing the recovery circuit before the arrival of each burst, a gating pulse for detecting an unmodulated signal appended to the header of each burst, and an aperture gating pulse for unique word detection. Since the arrival of a first burst after the period of no bursts may differ significantly from the instant which can be expected from the reference timing of the central station due to a satellite drift in orbital position, the timing at which the control signals are generated for the recovery circuit must be corrected according to the deviation of the round-trip propagation delay of the satellite communications system.
According to the technique currently employed in a satellite communications system, the central station sends a burst to all terminal stations in a broadcast mode by containing in it a time-slot assignment signal which assigns time slots to terminal stations having a message to transmit and a reference timing signal to allow the sending stations to establish the frame timing. The central station is provided with a telemetry circuit which includes a demodulator to receives its own burst returning from the satellite during the broadcast mode, a timing circuit that extracts the timing signal from the received burst and a framing circuit to establish the frame timing. A counter is provided which increments a binary count at clock intervals and resets it in response to the output of the framing circuit. The output of the counter is supplied to a control signal generator which generates control signals for application to the carrier and clock recovery circuit. Since the time slot position of the received burst offsets from the extracted timing signal by an amount corresponding to the satellite drift, the resetting of the counter is brought into correct timing relationship with the returning burst.
However, the telemetry circuit requires a separate frequency converter and must be duplicated for system reliability. Furthermore, the duplicated system requires protection switching circuitry, thus adding to the complexity and cost of the central station equipment.